Despite the increased awareness of the importance of Bacteroides in infectious processes, knowledge of the contribution of humoral and cellular factors to the resistance against infections caused by these anaerobic bacteria is limited. Studies will be performed to determine the requirements for immunoglobulin and the classical and alternative Complement pathways in normal human sera for opsonophagocytosis and intracellular killing of multiple strains belonging to different species of Bacteroides by human polymorphonuclear leukocytes. The investigation will also include studies to determine the distribution of natural and immune antibodies to Bacteroides among various classes of immunoglobulin, the opsonophagocytosis-promoting activity of the antibodies, the specificity of the antibodies, and the antigenic determinants of the cell envelope to which the antibodies are directed. In addition, purified components of the outer membrane complex of Bacteriodes will be tested for their ability to activate the classical and alternative complement pathways in normal human sera. The knowledge derived from this research will increase basic understanding of host resistance against Bacteroides and will be potentially valuable in the development of immunologic approaches for the diagnosis, prevention, and therapy of infections caused by these microorganisms.